1. Field of the invention
The present invention relates to a method for manufacturing a superconducting thin film, to a method for manufacturing a superconducting device and a superconducting thin film, and more specifically to a method for manufacturing a superconducting thin film having a non-superconducting region, such as an insulating layer, in it, a method for manufacturing a superconducting device which utilizes the superconducting thin film and the superconducting thin film manufactured thereby.
2. Description of related art
In order to produce superconducting devices which utilize an oxide superconductor, it is necessary to form an oxide superconductor thin film. For example, in order to prepare a tunnel type Josephson device which consists of SIS (superconductor-insulator-superconductor) junction, it is necessary to form a stacked structure which includes a first oxide superconductor thin film, an insulator thin film stacked on the first oxide superconductor thin film and a second oxide superconductor thin film stacked on the insulator film. Thickness of the insulator thin film of this SIS junction is determined by coherence length of the superconductor. Since an oxide superconductor has extremely short coherence length, the thickness of the insulator thin film of an SIS junction which utilizes an oxide superconductor should be less than few nanometers.
In the prior art, there has been proposed a three-terminal superconducting device having a channel of a superconductor formed between a source electrode and a drain electrode, so that a current flowing through the superconducting channel is controlled by a voltage applied to a gate electrode formed above the superconducting channel.
In order to control the current flowing through the superconducting channel by the voltage applied to the gate electrode, the superconducting channel of the above three-terminal superconducting device, so called super-FET, should be formed of an extremely thin superconductor film. Thickness of the extremely thin superconductor thin film is favorably about 5 nanometers, when the superconducting channel is formed of an oxide superconductor thin film. A gate insulator is often inserted between the superconducting channel and the gate insulator.
Both of the above mentioned tunnel type Josephson device and super-FET have a portion in which a superconducting layer and a insulator layer are stacked to each other. When the insulator layer is deposited on a portion of the superconductor layer, a photoresist is mainly used in order to mask portions on which the insulator layer should not be deposited. The photoresist is removed ultimately, however, fragments of the photoresist may remain so that the surface of the superconducting layer is contaminated. In case of oxide superconductor thin film, photoresist remover may react on the oxide superconductor thin film so that the surface of the oxide superconductor thin film loses its superconductivity and is also roughened.
If another thin film is deposited on the contaminated surface of the above oxide superconductor thin film, it is difficult to deposit crystalline thin film on this contaminated surface. Therefore, a stacked structure in which thin films are successively deposited on the oxide superconductor thin film does not have desired properties so that it is difficult to use the stacked structure including the oxide superconductor thin film for superconducting device.